In the Long Term Evolution (LTE for short) system, cyclic shift sequences of Zadoff-Chu (ZC for short) sequences are used as the preamble by the Random Access Channel (RACH for short). These cyclic shift sequences are also referred to as Zero Correlation Zone (ZCZ for short) sequences.
In practical systems, after a mobile phone is powered on, downlink synchronization is first performed, and then the detection of the Broadcast Channel (BCH for short) is initiated. A base station informs, via the BCH channel, the mobile phone of a logical index (Logical index) and the step length of the cyclic shift of the first ZC sequence available for the RACH of the current cell. According to the logical index, the mobile phone makes use of certain mapping rule to calculate a physical index (Physical index) of the corresponding ZC sequence, and then, generates usable ZCZ sequences according to the step length of the cyclic shift (and a certain “cyclic shift limitation rule” in case of high speed circumstances). If the number of the ZCZ sequences is smaller than a preset threshold P, the mobile phone automatically increments the sequence index, and continuously generates the ZCZ sequences using the next ZC sequence, until the total number of the ZCZ sequences is larger than or equal to P. Finally, the mobile phone randomly selects one sequence from all the generated usable ZCZ sequences as a preamble to be sent.
In practice, the mapping process between the logical indices and the physical indices of the ZC sequences is the process of re-sequencing the ZC sequences Wherein the generation formula of the ZC sequences is shown by Equation (1), wherein N is the length of the sequence, u is the physical index of the sequence, which refers to an index used in the generation of each ZC sequence, and gn(n) represents the sequence value of the nth sample point whose physical index is u. The index of the sequence is the sequence number of each ZC sequence in a queue of sequenced ZC sequences, where the ZC sequences are sequenced according to a certain criterion.
                                          g            u                    ⁡                      (            n            )                          =                  {                                                                                                                ⅇ                                                                        -                          j                                                ⁢                                                                              2                            ⁢                            π                                                    N                                                ⁢                                                  1                          2                                                ⁢                                                  un                          2                                                                                      ⁢                                                                                  ⁢                    when                    ⁢                                                                                  ⁢                    N                    ⁢                                                                                  ⁢                    is                    ⁢                                                                                  ⁢                    even                                                                                                                                          ⅇ                                                                        -                          j                                                ⁢                                                                              2                            ⁢                            π                                                    N                                                ⁢                                                  1                          2                                                ⁢                                                  un                          ⁡                                                      (                                                          n                              +                              1                                                        )                                                                                                                ⁢                                                                                  ⁢                    when                    ⁢                                                                                  ⁢                    N                    ⁢                                                                                  ⁢                    is                    ⁢                                                                                  ⁢                    odd                                                                        ,                          n              =              0                        ,            1            ,            …            ⁢                                                  ,                          N              -              1                                                          (        1        )            
FIG. 1 is a schematic diagram showing the frame structure of the Time Division Duplex (TDD) mode in the LTE system. As shown in FIG. 1, in this kind of frame structure, a radio frame of 10 ms (307200 Ts, 1 ms=30720 Ts) is divided into two half-frames, each half-frame is divided into 10 time slots of 0.5 ms, every two time slots compose a subframe of 1 ms, one radio frame comprises 10 subframes (which are numbered from 0 to 9), and one radio frame comprises 20 time slots (which are numbered from 0 to 19). For normal Cyclic Prefixes (CP) with lengths of 5.21 us and 4.69 us, one time slot comprises 7 uplink/downlink symbols of 66.7 us, wherein the length of the Cyclic Prefix of the first symbol is 5.2 us and that of the Cyclic Prefixes of the other 6 symbols is 4.69 us. For extended Cyclic Prefixes with a length of 16.67 us, one time slot comprises 6 uplink/downlink symbols. Additionally, in this kind of frame structure, the configuration of subframes has the following characteristics:                subframes 0 and 5 are fixed to be used in downlink transmission;        uplink/downlink switch with cycles of 5 ms and 10 ms is supported;        subframes 1 and 6 are special subframes which are used to transmit 3 special time slots, i.e. Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot Time Slot (UpPTS), wherein        the DwPTS is used for downlink transmission;        the GP is Guard Period and does not transmit any data;        the UpPTS is used for uplink transmission, comprising at least 2 uplink Single Carrier-Frequency Division Multiple Access (SC-FDMA) symbols for transmitting the Physical Random Access Channel (PRACH). For 5 ms uplink/downlink switch cycle, the UpPTS and subframes 2 and 7 are always used for uplink transmission; while for 10 ms switch cycle, the length of the UpPTS in subframe 6 is 0 and that of the UpPTS in subframe 1 can either be 0 or larger than 0.        
In the process of switching between uplink and downlink with a cycle of 5 ms, subframes 2 and 6 are fixed to be used for uplink transmission;
In the process of switching between uplink and downlink with a cycle of 10 ms, the DwPTS exists in two half-frames, the GP and the UpPTS exist in the first half-frame, the period of the DwPTS in the second half-frame is 1 ms, subframe 2 is used for uplink transmission and subframes 7 to 9 are used for downlink transmission. There are two types of PRACH in the TDD mode of the LTE system, wherein the first type is transmitted in non-special uplink subframes (subframes comprising special time slots are called as special subframe); and the second type is transmitted in the UpPTS. In the method for sequencing the first type of PRACH, all sequences are divided into two groups according to the Cubic Metrics (CMs) with a threshold of 1.2 dB, then in each group, sequences are divided into several sub-groups according to the maximum cyclic shift supported by the sequences in a cyclic shift limiting condition, and at last in each sub-group sequencing is conducted according to Cubic Metric values of the sequences. Cubic Metric (CM) is a kind of standard for measuring peak-to-average ratio of transmitting data, wherein the larger the CM is, the higher the peak-to-average ratio is. Its calculating method is as shown in the following formula (2):
                    CM        =                                                            20                ⁢                                  log                  10                                ⁢                                  {                                      rms                    ⁡                                          [                                                                        v                          norm                          3                                                ⁡                                                  (                          t                          )                                                                    ]                                                        }                                            -              1.52                        1.56                    ⁢          dB                                    (        2        )            wherein
                    v        norm            ⁡              (        t        )              =                                    v          ⁡                      (            t            )                                              rms        ⁡                  [                      v            ⁡                          (              t              )                                ]                      ,          ⁢            rms      ⁡              (        x        )              =                            (                                    x              ′                        ⁢            x                    )                M              ,v(t) is the amplitude of time domain signal, a cluster of discrete time domain sample points v(m) are obtained to simulate v(t) by performing sampling to time domain signal gu(n) of n=0, 1, . . . , N−1 at the time point of m=0, 1, . . . , M−1, M is the dimensionality of vectorX, and M>N.
It is obvious that the larger the dimensionality M of vectorx is, i.e. the more the sampling points are, the smoother the curve is, wherein the curve is obtained by imitating v(t) using discrete time domain sample points v(m), the higher the precision of the imitation is, and thus the higher the precision of the obtained CM values is.
Compared with the first type of PRACH, the second type of PRACH has broader sub-carriers, is more resistant against Doppler frequency offset and can solve the problem of frequency offset without using cyclic shift limit. Therefore, the method for sequencing the first type of PRACH is not applicable to the second type of PRACH. However, since the method for sequencing the ZC sequences applicable to the second type of PRACH has not been provided in the existing technologies, sequences with similar CM values can not be allocated to the same cell, with a result that different UEs using different ZC sequences in the same cell have different coverage, which restricts the flexibility of cell planning.